In thermal transfer printing, an image is formed on a receptor sheet by selectively transferring a dye from a dye donor sheet to a receptor sheet, typically having coated thereon an image receiving layer. The term "thermal transfer" is used herein in a broad sense and includes any printing process by which dye (alone or in association with carrier materials such as solvents, binders. etc.) is transferred from one layer to another layer or sheet upon an application of thermal energy. Typically, material to be transferred from the dye donor sheet is directed by a thermal printhead, which consists of small electrically heated elements (print heads). These elements transfer image-forming material from the dye donor sheet to areas of the dye receptor sheet in an image-wise manner.
Thermal transfer printing processes are well known in the art and include such systems as thermal dye transfer, sublimation dye transfer, and thermal mass transfer. Other systems included within the broad terminology used herein include direct transfer, strippable transfer, peel apart, and the like. In each of these systems, the dye is typically contained within a polymeric or resinous binder thereby forming a dye donor layer on the donor sheet. Typically, the dye is present in the binder in an amount of at least about 2%, depending upon the degree of its transfer and its covering power in the transferred state. The receptor sheet typically has a polymeric or resinous material coated thereon to form an image receiving layer for receipt of the dye. In a mass transfer system the dye is transferred with the binder. Thus, in such systems, the receptor sheet may not need an additional image receiving layer.
Thermal transfer dyes typically should meet several performance criteria for practical applications. They should be able to transfer from the donor to the receptor upon application of thermal energy. For systems other than mass transfer systems, in which the entire quantity of dye transfers, the dyes should ideally demonstrate grey levels, i.e., grey scales. That is, the dyes should be able to transfer in controlled amounts depending upon the amount of thermal energy applied to the donor. Also, thermal transfer dyes should be stable once incorporated into the donor sheet. That is, they should generally remain in solid solution without crystallizing or migrating to the surface. Furthermore, thermal transfer dyes should preferably be stable once transferred to the receptor sheet. That is, they should not significantly retransfer, e.g., resublime or retro-sublime, from the dye image on the receptor sheet.
Arylazoaniline dyes (often represented as A-N=N-E wherein A is a phenyl group and E is an aniline group) are commonly used in various printing systems. Examples of such dyes are disclosed in U.S. Pat. Nos. 4,059,402 (issued Nov. 22, 1977), 4,212,642 (issued Jul. 15, 1980), 4,614,521 (issued Sep. 30, 1986), 4,619,893 (issued Oct. 28, 1986), 4,776,886 (issued Oct. 11, 1988), and 4,975,410 (issued Dec. 4, 1990), as well as in W. Harms, Organofluorine Chemicals and their Industrial Applications, Chapter 9, p. 188, R. E. Banks, ed., (1979), G. Wolfram, Organofluorine Chemicals and their Industrial Applications, Chapter 10, p. 208, R. E. Banks, ed., (1979), and V. V. Karpov et al., Dyes and Pigments, 5, 285 (1984). Many of these dyes, however, are unsatisfactory for use in thermal transfer systems because they are generally unstable in either the donor layer or the receiving layer or both. That is, many of these dyes have a tendency to bloom or crystalize out of the donor layer and/or resublime from the receiving layer over time.
Arylazoaniline dyes useful for thermal transfer printing are disclosed in U.S. Pat. Nos. 5,122,499 (issued Jun. 16, 1992) and 5,234,887 (issued Aug. 10, 1993). In the '499 patent, it is generally mentioned that semi-polar substituents are used to provide a decreased degree of retro-sublimation; however, it is not clear what substituents are within this definition. Furthermore, emphasis is placed on substitutions on the amine nitrogen and the phenyl ring of the aniline component of the dye to provide acceptable properties. In the '887 patent the phenyl group is substituted by at least one electron-withdrawing group, e.g., CF.sub.3, but is free from groups carrying acidic hydrogen atoms capable of forming intermolecular hydrogen bonds. Typically, fluorine-containing dyes tend to bloom or crystallize out of the donor layer.